The present invention relates to lifting assemblies for boats and in particular to lifting assemblies of the type wherein an eye is mounted to the top deck and connected through a tension bearing member to a bow or keel mounted lifting strap.
In order to launch a boat or remove it from the water, it is common practice to hoist it above the water by means of hooks which engage lifting-eyes mounted to the deck of the boat. Because the deck is designed primarily to support loads acting in a downward direction such as, for example, forces exerted by persons walking on the deck, substantial damage would be done to the deck if an upward force were applied thereto, as would be the case if the boat were lifted by means of lifting rings mounted directly to the deck. To overcome this difficulty without foregoing the convenience of a lifting-eye located on the deck, the load is transferred to a heavily reinforced portion of the boat, such as the bow or keel, by means of a rod or chain connected between the deck ring and a bow or keel lifting strap. When the deck ring is pulled, the connecting member is tensioned so that virtually the entire force is transferred to the bow or keel.
One type of prior art lifting assembly comprises a lifting strap adapted to be bolted to the bow or keel of a boat, a solid metal rod pivotally connected at one end thereof to the strap and connected at the other end to a deck mounted lifting-eye. The strap comprises a metal strip bent to comprise a flat portion having a pair of apertures therethrough and a slotted, elongated loop portion. The rod is connected to the lifting strap by means of a T-nut which is slidably and rotatably received in the loop portion and includes an internally threaded socket coaxial with the longitudinal dimension of the nut.
The primary difficulty with this type of device lies in the irregular shape of the T-nut, which is a single casting, and has a substantial number of inherent weak points that enhance crack formation and propagation when the nut is stressed. Obviously, the greatest stress is applied when the boat is lifted so that failure of the nut at this point generally results in the boat being dropped thereby causing substantial damage to the boat and possible injury to nearby persons. The faulty design inherent in this type of nut has resulted in an unacceptably high incidence of such accidents.
Another design deficiency in this assembly lies in the shape of the loop portion of the strap which is elongated and considerably larger than that portion of the T-nut which is contained therein. As the T-nut is pulled against one end of the loop portion during lifting, there is a tendency for the loop to flatten out and elongate so that not all of the lifting forces are transferred to the bow or keel. This results in the deck being pulled upwardly away from the bow and keel with resultant damage since it is not designed to withstand lifting loads.
A further example of prior art lifting assemblies is disclosed in U.S. Pat. No. 3,863,588 to one of the present applicants. It comprises a pair of U-bolts extending through the deck and bow, respectively, and secured thereto by nuts on opposite sides of the deck and bow which clamp escutcheon plates thereagainst. The U-bolts are interconnected by an adjustable tension element, which may be a chain or strap, connected at opposite ends to eye-bolts which in turn are secured to the deck and bow U-bolts. The drawbacks to this type of device include stretching and flexing of the chain or strap and off-center twisting forces being applied to the lifting-eyes, especially the bow mounted eye.